There have recently been advances in miniaturization in the field of electronic apparatuses. There have also been advances in the fabrication of positive temperature coefficient thermistors in chip form. For example, multilayer positive temperature coefficient thermistors are known as positive temperature coefficient chip thermistors.
A multilayer positive temperature coefficient thermistor of this type usually includes, for example, a ceramic body in which BaTiO3-based semiconductor ceramic layers and internal electrodes are alternately stacked and includes external electrodes arranged on both ends of the ceramic body, the external electrodes being electrically connected to the internal electrodes, in which adjacent internal electrodes in the stacking direction with semiconductor ceramic layers extend alternately to opposite ends of the ceramic body.
As a conductive material constituting the internal electrodes, an inexpensive base metal material with good conductivity, such as Ni, is widely used. The ceramic body is formed by applying a conductive paste containing a base metal material to ceramic green sheets to be formed into semiconductor ceramic layers by screen printing to form conductive layers, stacking the ceramic green sheets including the conductive layers, sandwiching the resulting stack between ceramic green sheets not including a conductive layer, performing press bonding, and performing co-firing in a predetermined atmosphere. If co-firing is performed in an air atmosphere, the base metal material such as Ni is readily oxidized. Thus, the co-firing is performed in a reducing atmosphere.
Meanwhile, the semiconductor ceramic layers are also reduced in the case where the semiconductor ceramic layers and the internal electrodes are co-fired in a reducing atmosphere as described above, so that a sufficient rate of resistance change is not provided. Thus, attempts are made to ensure a desired rate of change of resistance by performing a reoxidation treatment in an air atmosphere or oxygen atmosphere after the co-firing is performed in a reducing atmosphere.
In the reoxidation treatment, it is difficult to control the heat treatment temperature and to allow oxygen to penetrate into the middle portion of the ceramic body, thereby readily resulting in uneven oxidation. Thus, a sufficient rate of resistance change may fail to be obtained.
For example, Patent Document 1 discloses a ceramic electronic component including a ceramic body impregnated with a glass component and electrodes arranged on surfaces of the main body of the electronic component, in which the ceramic body has a relative density of 90% or less.
In Patent Document 1, a reduction in the sintered density of the semiconductor ceramic layers and an increase in the porosity of the semiconductor ceramic layers seem to facilitate the penetration of oxygen into the middle portion of the ceramic body, thereby ensuring a desired rate of resistance change.
When a multilayer positive temperature coefficient thermistor is mounted on a substrate, the thermistor is usually soldered to the substrate by reflow heat treatment. A high porosity of the semiconductor ceramic layers as in Patent Document 1, however, can cause the penetration of the flux contained in solder into the ceramic body through pores in the semiconductor ceramic layers located on surfaces of the ceramic body, thereby reducing the withstand voltage.
In Patent Document 1, the impregnation of the ceramic body with the glass component results in the formation of glass films in the pores in the surfaces of the ceramic body, thereby preventing the penetration of the flux into the ceramic body.
Patent Document 2 discloses a prior art arrangement in which semiconductor ceramic layers have different porosities. Specifically, a multilayer positive temperature coefficient thermistor is reported in which among a plurality of thermistor layers serving as effective layers arranged between two outermost internal electrodes in the stacking direction, the thermistor layers located in a middle portion in the stacking direction have a higher porosity than that of thermistor layers located outside the middle portion in the stacking direction.
In Patent Document 2, a binder containing an organic material such as polystyrene particles is used, and different ceramic green sheets having different organic material contents are formed. The different ceramic green sheets having different organic material contents are stacked in such a manner that the ceramic green sheets located in the middle portion of a ceramic body in the stacking direction have relatively high organic material contents and that the ceramic green sheets located at outer portions of the ceramic body in the stacking direction have relatively low organic material contents. The resulting stack is subjected to a firing treatment to burn off the organic material, thereby forming pores. Thus, the thermistor layers located in the middle portion of the ceramic body in the stacking direction have a higher porosity than that of the thermistor layers located at the outer portions in the stacking direction. In other words, the thermistor has a structure in which the porosity of the semiconductor ceramic layers located on surfaces of the ceramic body is lower than that in the middle portion of the ceramic body.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2002-217004
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-93574